Currently, golf balls are generally classified based on their constructions as either a one-piece, two-piece or three-piece ball. The difference in play characteristics resulting from these different types of constructions can be quite significant. The one-piece ball typically comprises a homogenous mass of polybutadiene, monomers, fillers, antioxidants, curing agents, and the like. These types of balls are generally inexpensive and durable, but do not provide superior distance or spin. Typically one-piece balls are manufactured for used as range balls or practice balls.
Often, it is desirable to select a relatively "hard" material such as an ionomer resin as the cover material so as to impart a high degree of durability to the golf ball. Two piece balls are made with a solid core, usually made of a polymeric or crosslinked rubber material which is encased by a cover material. Typically the solid core is a high-energy acrylate or resin and is covered by a tough, cut-proof blended cover. The cover material can be formed from any one of a number of conventional materials including ionomer resins such as SURLYN.RTM. (Du Pont), or an elastomer. The combination of the core and cover materials provide a "hard" ball that is very durable. Further, such a combination imparts a high initial velocity to the ball which results in improved distance. Because these materials are very rigid, two piece balls have a hard "feel" when struck with a club. Likewise, due to their hardness, these balls have a relatively low spin rate which makes them difficult to control, particularly on shorter approach shots. Nonetheless, balls having a two piece construction are generally most popular with the average recreational golfer because they provide a very durable ball while also providing maximum distance.
Some three-piece balls are referred to as "wound" balls due to their construction. Wound balls typically have either a solid rubber or hollow liquid center core around which many yards of a stretched elastic thread or yarn-like material is wound. Typically, the hollow liquid filled center core is made of a thermoset material such as a natural or synthetic rubber or blends thereof. The wound core is then covered with a durable cover material such as a SURLYN.RTM. or similar material or a softer cover such as balata or polyurethane. Wound balls are the preferred ball of the more advanced players due to their superior spin and feel characteristics. Wound balls are generally softer and provide more spin than a one or two piece ball which enables a skilled golfer to have more control over the ball's flight. Moreover, as a result of their more complex structure, wound balls generally require a longer time to manufacture and are more expensive to produce than a two piece ball.
Initially, wound balls were manufactured having a solid center core. However, it was eventually discovered that wound balls with a liquid-filled center provided superior playing and "feel" characteristics. Accordingly, a number of methods for producing liquid filled centers are disclosed in the prior art. For example, in one method, a suitable liquid is encapsulated within a gelatin capsule to form a liquid filled sphere. This sphere is then frozen, covered with two preformed hemispherical rubber shells and placed in a heat mold. In the mold, the rubber shells are vulcanized to provide adequate strength to the sphere so it can be used as a liquid filled center.
Another method includes placing a liquid or paste material into a spherical mold and freezing the material to form a solid core material. A non-vulcanized rubber coating is subsequently placed over the frozen sphere and vulcanized to form a rubber outer shell which retains the liquid material. A variation of this method is described in U.S. Pat. No. 3,490,770, wherein a spherical mold is filled with a liquid material which is convertible to a solid molecular structure by, e.g., thermosetting, polycondensation, catalytic conversion, or by other chemical conversion mechanisms. The mechanism for solidification is employed so as to provide a solid skin which encloses the remaining, unreacted liquid.
Still another method of creating a liquid filled center involves vulcanizing two rubber hemispheres, applying an adhesive to the outer "lip" of the hemispheres and assembling the two hemispheres to create a sphere which is subsequently vulcanized. Liquid is then injected into the sphere through the use of a hypodermic needle and the resulting puncture hole is sealed. Typically a patch, with or without an adhesive material is used to seal the hole. Additionally, it is also known to insert a conical shaped plug into the hole wherein the plug is secured with an adhesive or some other means in order to seal the hole. Such adhesives tend to be difficult to apply and the surface area they cover must be completely free of contamination for proper adhesion.
Alternatively, as described in U.S. Pat. No. 4,943,332, the hemispheres can be submerged in a desired liquid before the two halves are joined and either vulcanizing the sphere while submerged or upon removal from the liquid.
In one conventional commercial method of producing liquid centers for golf balls, the center is made from a natural rubber compound, wherein the base rubber material and other ingredients are mixed on an open mill. After the rubber stock is thoroughly mixed, the stock is placed in a calender which forms it into flat sheets. One sheet is placed into a die in which recessed cups are formed. A vacuum is placed on the die in order to pull the sheet into the cups.
A second sheet is then placed onto a vacuum die plate which opposes the first die. A small amount of water is sprinkled on top of the first sheet. The die assembly is then closed, pinching off bladders which consist of the two circular discs of rubber which are crimped at the edges and have a few drops of water trapped in the middle. The bladders are removed from the rubber sheets and placed in a bold which has hemispherical cavities defining the desired shape of the center. The mold is then heated, causing the trapped water to turn to steam, which in turn inflates the bladders against the mold wall. Continued heating causes the rubber to vulcanize into the desired hollow shape.
Subsequently, the center is filled by puncturing it with a hypodermic needle and injecting a fluid of a controlled density. The needle is then removed and the center is washed to remove any excess filling solution. The hole is plugged with an adhesive material which crosslinks into a solid when exposed to ultraviolet light. At this point, the liquid filled center construction is complete and ready for the winding phase of the golf ball construction.
Very few materials suitable for use in the shell of the liquid filled center, especially for use in a wound ball, are disclosed in the prior art. For example, U.S. Pat. No. 4,943,332 discloses the use of conventional rubbers such as natural rubber, butadiene rubber, isoprene rubber or a mixture thereof. Likewise, U.S. Pat. No. 4,904,320 discloses that the center shells or envelopes are usually made of a natural rubber. Additionally, U.S. Pat. No. 4,244,855 discloses a center core formed of a non-cross-linked, butadiene-styrene radial block copolymer and a large portion of filler material. However, natural rubber, synthetic rubber and blends thereof remain the industry standard as material for the liquid filled center shell.
U.S. Pat. Nos. 5,150,906 and 5,480,155 are both directed towards non-wound golf balls wherein a relatively large diameter, pre-formed shell rather than a core is the starting point for the balls. Both of these patents explicitly distinguish golf balls which are made by forming a center core, such as a liquid-filled rubber bladder, and then molding a layer or a winding around the center core. Instead, these patents disclose forming a golf ball from a shell into which a liquid is injected to form the core material, wherein the shell may be the outer layer of the ball or may be slightly smaller than the final diameter of the ball and have a cover thereon. The liquid material may either remain a liquid or subsequently react to form a solid core. The shell may be formed out of any number of materials, including a variety of thermoplastic materials. The hole through which the liquid is injected is plugged with a conical plug which is secured by ultrasonic, vibrational laser or solvent bonding, compression molding or adhesive bonding.
Presently, conventional processes for producing a liquid filled center core are rather complicated and labor intensive, requiring a number of distinct and separate process steps. For example, conventional processes typically require that the steps of milling, calendaring, forming and molding the hollow center cores all be performed separately. Due to the complicated nature of the conventional processes, the manufacture of the liquid-filled centers is very time consuming and labor intensive. Accordingly, due to the large amount of labor and time involved, conventional liquid filled centers are relatively expensive to produce.
The difficulty in producing the conventional liquid filled centers is directly related to the materials from which the center shells are typically made. As discussed above, natural and synthetic rubbers and blends thereof are overwhelmingly used in conventional liquid filled spheres. These materials are generally described as "thermosetting" materials, a term generally used to describe polymeric materials which solidify or "set" irreversibly when heated or exposed to radiation or a chemical reaction.
Thermosetting polymers consist of linear chains having numerous functional groups or double bonds distributed along the chains, which are capable of forming crosslinks on further polymerization reactions. This may occur upon exposure to heat or other radiation or by adding catalysts, resulting in a three-dimensional network that makes the polymer infusible and insoluble. Since the crosslinks are strong covalent bonds that can only be broken at high temperatures at which the organic polymer would be completely decomposed, the thermosetting polymer, once it is fully crosslinked, is infusible and insoluble for all practical purposes; it cannot be remelted, remolded or dissolved. Thus, the methods for making center core shells from thermosetting materials typically require several additional processing steps due to the physical and chemical properties of the thermoset materials.
Accordingly, a need exists for materials which are not thermosetting, such as thermoplastic materials, which are suitable for use as center core shell materials. Additionally, it would be desirable if the physical properties of the materials for use as center core shells were such as to facilitate a simplification of the processes employed to produce the liquid filled golf ball center. Further, a need exists for simpler and more commercially practical methods for making liquid filled golf ball centers and golf balls employing such centers.